The ten helical twist angles of B-DNA.

On the assumption that the twist angles between adjacent base-pairs in the DNA molecule are additive a linear system of 40 equations was derived from experimental measurements of the total twist angles for different pieces of DNA of known sequences. This system of equations is found to be statistically consistent providing a solution for all ten possible twist angles of B-DNA by a least squares fitting procedure. Four of the calculated twist angles were not known before (tau AC, tau AG, tau CA, tau TA). The other six twist angles calculated are very close to the experimentally measured ones (tau AA, tau AT, tau CC, tau OG, tau GA, tau GC). The data used were obtained by the electrophoretic band-shift method (1-3), crystallography (4) and nuclease digestion of DNA adsorbed to mica or Ca-phosphate surface (5,6). The validity of the principle of additivity of the twist angles implies that the angle between any particular two base-pairs is a function of only these base-pairs, independent of nearest neighbours.